Acrolein may be obtained by oxidizing propylene. In the preparation of acrylic acid by vapor phase catalytic oxidation of propylene, there have been the following three manners of carrying out the reaction:
1. One-step oxidation process wherein propylene is oxidized directly into acrylic acid in the presence of a catalyst.
2. Two-step process wherein acrolein is mainly prepared in the first oxidation step, acrolein is then separated from by-products such as acrylic acid and acrolein is oxidized in the second oxidation step. 3. A process wherein acrolein is mainly prepared in the first oxidation step and oxidized in the second oxidation step without separating acrolein from by-products or off gas (hereinafter this process will be referred to as the continuous process).
The present invention can be applied to either process (2) or (3). Particularly, if the present invention is applied to the continuous process, the merit that the yield of by-produced propionic acid can be reduced while the high yield of acrylic acid is kept can be obtained positively. Though acrylic acid obtained by vapor phase catalytic oxidation of propylene contains a small amount of propionic acid, separation of propionic acid therefrom is quite difficult by either a physical process such as distillation or by a chemical treatment, since they have nearly the same physical properties (such as boiling point) because the molecular weight of propionic acid is very close to that of acrylic acid, and since their chemical properties are very close to each other because their chemical structures are not so different from each other.
Further, the presence of propionic acid or an ester thereof which cannot be polymerized exerts a great influence upon the quality of acrylic acid or an ester thereof.
After investigations of reducing the by-production of propionic acid in the oxidation step, the inventors have confirmed that propylene unreacted in the first oxidation step and contained in the off gas is catalytically oxidized into propionic acid in the second oxidation step on a catalyst mainly comprising molybdenum and vanadium. Further a very small amount of propylene is converted into propionaldehyde in the first oxidation step and the propionaldehyde is converted into propionic acid in the second oxidation step.
Molybdenum-vanadium catalysts used for the preparation of acrylic acid from acrolein have been previously described in Japanese Pat. Publication Nos. 1775/1966, 1662/1967, 12129/1969, 12886/1969 and 26287/1969, etc. Those molybdenum-vanadium catalysts for the oxidation of acrolein have a poor activity for propylene at a reaction temperature suitable for the oxidation of acrolein. For example, under conditions for oxidizing more than 90% of acrolein, about 10% of propylene is oxidized.
The propylene, though in a small amount, is converted partially into propionic acid.
Thus, in case acrylic acid is to be prepared from propylene by the continuous process, if 100% of propylene is oxidized in the first oxidation step, propionic acid prepared will be in only a very small amount. However, 100% conversion of propylene causes a great reduction in selectivity to acrolein. Therefore, for preventing reduction in single pass yield of acrolein, conversion of propylene must be controlled to about 95 - 97% . Consequestly, 3 - 5% of propylene is introduced in the second oxidation step in this case.